The present invention relates generally to dispensing systems for applying a liquid material and, more particularly, to dispensing systems using air to fiberize a dispensed strand or filament of material before contacting a moving substrate. Any liquid dispensing system such as a meltblowing system comprises one or more modules, each having a die tip for applying the liquid material.
Various systems for dispensing liquids are well known and can use the present invention. These include, but are not limited to airless spray systems which apply a liquid to a moving web or substrate, systems which use dispensed air to move or fiberize the dispensed liquid before contacting a non-moving substrate and systems which use dispensed air to move or fiberize the dispensed liquid before contacting a moving substrate. The present invention will be described in detail with respect to the latter type of system, and in particular a meltblowing dispensing system. Meltblowing dispensing systems have been developed for applying viscous material such as polymer material onto a moving substrate to form nonwoven fabric, and, more recently, for applying a pattern of hot melt adhesives onto a moving substrate during the manufacture of various products, such as diapers, feminine hygiene products, multiply tissues and the like. In general, meltblowing systems include a source of liquid material, a source of air, a manifold for distributing the liquid material and air, at least one and usually a plurality of modules mounted to the manifold for receiving the liquid material and air and dispensing an elongated filament of the liquid material which is attenuated and drawn down by the air before being randomly applied onto the substrate. Each module of the meltblowing system comprises a liquid material passage and an air passage and a die tip or nozzle. In general, a meltblowing die tip comprises a plurality of liquid material orifices or outlets arranged in a row and a slot on each side of the row of material orifices for dispensing the air. Instead of two slots, it is well known to use two rows of air orifices or outlets parallel to the row of material orifices.
One known type of meltblowing die tip used in applying hot melt adhesive material comprises a triangular nosepiece extending outwardly from the die tip and towards the substrate. The nosepiece is defined by a pair of converging walls which meet at an apex. A series of spaced adhesive orifices or outlets are aligned along the apex to dispense filaments of hot melt adhesive toward the substrate. Parallel rows of air discharge orifices or outlets are formed on opposite sides of the nosepiece to discharge jets of high velocity air. In each row there can be one or more than one air orifice associated with each adhesive orifice. The air orifices are typically elliptical in shape and formed adjacent the base of the triangular nosepiece with the air discharged generally parallel to the converging walls of the nosepiece. Of course, the air orifices can be other geometric shapes and the air can be channeled up the sides of the nosepiece through the use of an air plate to effectively place the air outlet adjacent to the adhesive orifice. The high velocity air jets on the opposite sides of the nosepiece are directed toward the dispensed filaments to draw down and attenuate the filaments to a reduced final diameter. The filaments of hot melt adhesive are deposited on the surface of a moving substrate to form an adhesive layer thereon onto which may be laminated another layer.
During the manufacture of multi-ply tissues, for example, a ply of tissue paper is conveyed past the meltblowing die which deposits a layer of fine adhesive fibers on the tissue paper before it is bonded to another ply. As the tissue paper moves toward the die tip, the air between the tissue and the die tip is agitated and moved in the same general direction as the tissue. In addition, due to the high velocity of the air being discharged from the air orifices, the air in the area adjacent the air orifices is aspirated toward the air orifice. This air between the tissue and die tip, referred to as entrained air, has particles of dust and other materials suspended in it. This is particularly prevalent in the environment where ply bonding is done, but is present to a lesser degree in any environment where material dispensing is done. This moving entrained air combines with the high velocity air to form process air which attenuates and draws down the filament of dispensed hot melt adhesive. However, the movement of the entrained air is not uniform in velocity or direction over the length of the die tip and this causes, to a lesser degree, the process air to also be non-uniform. The focus of the pattern of the filament applied to the moving substrate is negatively affected by the non-uniformity of the process air. In addition, the entrained air is drawn against the die tip which has been made tacky by the adhesive vapor or mist released during the normal melting of adhesives, and eventually, the dust particles build up against the face of the nosepiece and between the air discharge orifices until one or more of the high velocity air orifices are partially or completely blocked. As this build up of dust particles occurs, the balance of air pressure across the meltblowing die tip is disturbed and the nonuniformity of the process air increases, thus creating a less focused pattern of adhesive filaments on the tissue paper.
To combat the dust build up problem that occurs during the ply bonding process, manufacturers of paper products, for example multiply tissues, have invested in costly dust control systems to control the amount of dust in the vicinity of the adhesive dispensing system. While such control systems reduce the amount of dust in the air, the air orifices still become clogged or stopped and the adhesive dispensing system must still be taken off-line, so that the operator can take the appropriate maintenance actions. In addition, dust control systems are generally expensive and add to production costs. Thus, there is a need for an adhesive spraying die tip that increases the uniformity of the process air and is less susceptible to dust build up that may partially or completely block one or more of the high velocity air orifices.
The present invention is a die tip for use in a module or system for dispensing liquid material. The die tip of the present invention can be used in various liquid dispensing systems, but will be described herein as a die tip for use in a meltblowing system for applying a hot melt adhesive onto a moving substrate. The die tip improves the uniformity of the process air used to attenuate and draw down the dispensed filament of material and correspondingly, improves the consistency or focus of the pattern of the dispensed liquid. The die tip also minimizes the accumulation of dust and other particles around the melt and air discharge orifices caused by the motion of the entrained air between the die tip and the substrate. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.
A meltblowing die tip in accordance with the preferred embodiment of the present invention includes a base member having a triangular nosepiece extending outwardly therefrom and toward the moving substrate. The nosepiece includes a pair of converging walls that terminate in an apex extending the full width of the base member. A series of liquid dispensing orifices or outlets are formed in spaced relationship along the apex for dispensing filaments of liquid material, such as hot melt adhesive, toward the substrate.
A pair of slots or a series of air discharge orifices or outlets are formed in spaced relationship in a pair of parallel rows disposed along opposite sides of the nosepiece. The air discharge outlets discharge high velocity air jets toward the filaments dispensed from the liquid dispensing outlets. The high velocity air jets draw down and attenuate the filaments to a reduced final diameter before they are deposited at random on the moving substrate.
In accordance with the present invention, an air foil extends outwardly from a lower surface of the base member and toward the moving substrate. The air foil preferably comprises a pair of air deflecting members extending the full width of the base member and positioned outboard of, and generally parallel with, the rows of air discharge outlets. The air deflecting member positioned upstream of the nosepiece has a radius or otherwise curved outer surface that deflects the entrained air moving toward the die tip, due to aspiration caused by the high velocity air and to air movement caused by the moving substrate, in a direction away from the air discharge outlets and at least partially toward the moving substrate, and creates a vortex that provides a positive air flow in a direction away from the air discharge outlets. The air deflecting member positioned downstream of the nosepiece has a radius or otherwise curved outer surface that deflects the entrained air moving toward the die tip, due to aspiration caused by the high velocity air, in a direction away from the air discharge outlets and at least partially toward the moving substrate and creates a vortex that provides a positive air flow in a direction away from the air discharge outlets. As a result of the entrained air being directed away from the air discharge outlets, the uniformity of the process air, high velocity air and entrained air combined, is increased, since the entrained air combines with the high velocity air at a point further away from the air discharge outlets than would otherwise occur which reduces the effect of the moving entrained air on the high velocity air, and the amount of dust or other particles built up at the die tip is reduced. Thus, the air deflecting members improve efficiency and focus of the pattern of the filaments applied to the moving substrate.
In the preferred embodiment, the air deflection members are symmetrical and are formed by making a pair of substantially identical parallel elongated slots on opposite sides of the nosepiece. Each slot intersects a row of air flow passages extending through the base member on opposite sides of the nosepiece to form the air discharge outlets at terminal ends of the air flow passages. The air discharge outlets on each side of the nosepiece lie in a common plane defined by a recessed wall of each slot. The plane defined by each recessed wall is substantially transverse to the axes of the air flow passages on each side of the nosepiece. In this way, the air discharge outlets are formed with a circular shape and are configured to discharge the high velocity air jets generally parallel to the converging walls of the nosepiece. The circular-shaped air discharge outlets improve the behavior of the high velocity air jets and also increase air efficiency over elliptical air discharge outlets of the past.
Further, the meltblowing die tip of the present invention reduces the need for expensive dust removal systems in the vicinity of the meltblowing dispensing system and reduces the amount of maintenance required to keep the meltblowing die tips clean and operational. The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.